1. Field of the Invention
The present invention relates to a solder ball land metal structure of ball grid array semiconductor packages, and more particularly to a solder ball land metal structure which is configured to have etching holes or a tooth-shaped portion, thereby obtaining a maximum contact area between a solder ball land metal element (hereinafter, simply referred to as "land metal element") and a solder ball fused on the land metal element to prevent the solder ball from being separated from the land metal element.
2. Description of the Prior Art
In accordance with high integration techniques associated with semiconductor chips, an increased number of circuits can be integrated on a semiconductor chip having a limited size. Such a semiconductor chip has an increased number of input/output signals. In order to use such a semiconductor chip, a ball grid array (BGA) semiconductor package has been developed which has an increased number of input/output pins at an increased mounting density. Such a BGA semiconductor package is provided with solder balls which are fused on one surface of the semiconductor package. The solder balls serve as input/output terminals. By virtue of the use of such solder balls, it is possible to accept an increased number of input/output signals as compared to conventional semiconductor packages. In particular, the BGA semiconductor package has a chip size not so large as compared to conventional packages. In this regard, the BGA semiconductor package has been highlighted as a next generation semiconductor package.
Now, a general configuration of such a BGA semiconductor package and a fused structure of its solder balls will be described in conjunction with FIGS. 1, 2A and 2B.
Referring to FIG. 1, a general configuration of a BGA semiconductor package is illustrated. As shown in FIG. 1, the BGA semiconductor package includes a semiconductor chip 400 which is centrally bonded to the upper surface of a substrate 300 made of bismaleimide triazine epoxy resin (hereinafter, such a substrate is referred to as "BT substrate") by means of an epoxy layer 500. The semiconductor chip 400 has input/output pads 410 connected to metal traces 140 formed on the outer portion of the upper surface of the BT substrate 300 by means of wires 600. A solder mask 200 is formed on the metal traces 140 to protect a circuit pattern configured by the metal traces 140. Such metal traces 140 and solder mask 200 are also laminated on the lower surface of the BT substrate 300. A plurality of land metal elements. 100 are formed on the lower surface of the BT substrate 300. The land metal elements 100 are connected to the metal traces 140, respectively. Solder balls 150 are fused on the land metal elements, respectively. The solder mask 200 disposed at the lower surface of the BT substrate 300 is provided with recesses for receiving the metal traces 140 and land metal elements 100, respectively, in order to protect a circuit pattern configured by the metal traces 140. Although not shown, each metal trace 140 disposed at the upper surface of the BT substrate 300 is connected with each associated metal trace 140 disposed at the lower surface of the BT substrate 300. In order to protect the semiconductor chip 400 and wires 600 from the environment, a seal 700 is molded on the upper surface of the BT substrate 300. Thus, an one-side molding structure is obtained.
In such a BGA semiconductor package, regions, on which solder balls are fused, are called "land metal elements". Such land metal elements are mainly classified into those of a solder mask defined (SMD) type and those of a non-solder mask defined (NSMD) type respectively shown in FIGS. 2A and 2C. Conventional fused structures of solder balls on land metal elements will be described in conjunction with FIGS. 2A and 2C.
FIG. 2A is a plan view illustrating an SMD type land metal element in a state prior to the fusing of a solder ball. As shown in FIG. 2A, a solder mask 200 covers a metal trace 140 and the outer edge portion of a land metal element 100 connected to the metal trace 140. This will be described in detail in conjunction with FIG. 2B which is a cross-sectional view taken along the line 2B--2B of FIG. 2A. The land metal element 100, which is made of copper, is formed on a BT substrate 300. A nickel (Ni) film 110 and a gold (Au) film 120 are sequentially plated on the land metal element 100. The land metal element 100 is disposed in such a manner that its outer edge portion is interposed between the BT substrate 300 and solder mask 200.
The land metal element 100 is subsequently processed in a furnace maintained at a high temperature so as to fuse a solder ball 150 on its plated surface. The reason why the plated surface is provided on the land metal element 100 is to allow the solder ball 150 to be easily fused on the land metal element 100. The gold film 120 plated on the land metal element 100 is melted along with the solder ball 150 during a fusing process, so that it is mixed with the melted solder ball 150. As a result, the solder ball 150 is fused on the nickel film 110.
On the other hand, FIG. 2C is a plan view illustrating an NSMD type land metal element in a state prior to the fusing of a solder ball. As shown in FIG. 2C, a solder mask 200 covers only a metal trace 140 so that a land metal element 100 connected to the metal trace 140 is completely exposed through an opening 210 of the solder mask 200. This will be described in detail in conjunction with FIG. 2D which is a cross-sectional view taken along the line 2D--2D of FIG. 2C. The land metal element 100 is formed on a BT substrate 300 inside the opening 210 of the solder mask 200. A nickel film 110 and a gold film 120 are sequentially plated on the land metal element 100. The land metal element 100 is subsequently processed in a furnace maintained at a high temperature so as to fuse a solder ball 150 on its plated surface. As in the structure of FIG. 2A, the solder ball 150 is melted along the gold film 120 plated on the land metal element 100 during a fusing process, so that it is mixed with the melted gold. As a result, the solder ball 150 is fused on the nickel film 110.
However, both the SMD and NSMD type land metal structures have problems which should be solved. Conventional techniques associated with land metal structures have been focused on the purpose for preventing solder balls fused on land metal elements from being subsequently separated from the land metal elements. Since solder balls in a BGA semiconductor package serve as a medium for exchanging input/output signals between a semiconductor chip and a mother board, the BGA semiconductor package completely loses its function when the solder balls are separated from the land metal elements. In other words, such a solder ball separation phenomenon considerably deteriorates the reliability of the BGA semiconductor package.
In order to reduce such a solder ball separation phenomenon while allowing a solder ball to be easily fused on a land metal element, conventional techniques involve a process for sequentially plating nickel and gold films on a smooth and flat surface of the land metal element. Although the plated films provide a mechanism capable of easily fusing the solder ball on the land metal element, they can not avoid the solder ball separation phenomenon.
In the case of the SMD land metal structure, solder balls are easily separated from land metal elements at a mixed layer between the solder ball and the nickel of the nickel-plated film. In the case of the NSMD type land metal structure, solder balls are easily separated from the BT substrate well as from the land metal elements.